I need some advice regarding the connection of an electric underfloor heating system to the main circuit.

The plan is to lay a 200w system under the tiles of a 30cubic metre conservatory and expect to connect this through a single thermostat and contactor to the mains.

I will employ a qualified electrician to do the electrical installation, but at this stage I am seeking advice because I am getting conflicting views from the builder and floor tile supplier regarding how the connection can be made and a different view on the maximum size (wattage) of system I can lay. The builder is being conservative and suggests a low powered option connected to the consumer unit (through the thermostat only).

The heating system is quoted within the total build which the builder will install. Investment in materials is high and so to is the risk of getting an useless heating system. I therefore need to make sure that the buldier's advice is correct so that if needed I can make arrangements to employ an independent electrician for the electrical installation and leave the builder to lay the heating mat etc, or be better informed to instruct the builder.

A factor in the builder's advice is that the current consumer unit (11way) is full (house, garage, external building etc) and he seems to plan to connect the system to the ring main.

Questions therefore are:
1. Would my ideal system connected through a contactor be realistic?
2. How would this contactor be connected to the consumer unit (are bigger units available to accommodate the use of the contactor, can a separate housing unit be connected to the consumer unit etc)?

The size you talk about 200W is only 1 amp in fact a little less so connecting to ring main no real problem although in theory all fixed appliances should have there own circuit. It does not really need a contactor there are special thermostats for under floor heating which slide down a tube to be between mat under the floor and connect to the control unit about 1 meter up the wall. The length of the thermostat and cold tails limit where the control unit can be placed. Until the tiles are laid over the heating wires they are very easy damaged and careful liaison between builder and electrician is required to ensure they are not damaged and I would let the builder employ the electrician so if something does go wrong you only have to deal with one person.
My father had it laid in a wet room and the builder damaged the cables and all the tiles had to be removed and a new one laid. It dries the floor OK as it was designed to but any hotter it would burn your feet and it needs a towel rail as well to heat the room. Also very slow to warm up and cool down so has to be on at least two hours before going for a shower and shower water cools it so another hour after the shower to dry floor and my mother who it was done for gets very little benefit as she is in a wheel chair and only dabs at the floor with her one remaining leg. Hardly every used now only glad the towel rail went in as well.
Yes you can get consumer units which will take contactors and bell transformers on same DIN rail as MCB/RCBO’s
Eric

i lay this underfloor heating about twice a month, firstly check regs there is now a large section on electric underfloor heating, you must leave a plan of the matt and stat locations at consumer unit, etc etc. i spur off ringmain into switched fuse spur, then use 1.5 t&e to thermostat, run thermostat probe in conduit so if it fails at a later date can easily be pulled back and replaced,
prime floor with acrilic primer, lay matt, check continuity of matt, apply self leveling compound to matt to screed in, once dry check continuity again then again once tiles layed, all these readings have to be left in manufacturers guarantee booklet and left at consumer unit.